Staff markup was enabled on all block types in
https://github.com/edx/edx-platform/pull/10903

This works well inside the courseware, but it breaks layout of the course about
page, which is also an XModule, see:
https://github.com/edx/edx-platform/pull/10903#issuecomment-164266342

This commit disables staff markup/staff debug on all blocks except
blocks tagged with 'detached'. Detached blocks include course about
and info pages, static tabs.